This work focuses on the characterization of a novel protein, Soc2, that interacts genetically with the primary cyclin dependent kinase in fission yeast, Cdc2. Soc2 is a 66kDa essential protein that contains a ubiquitin associated (UBA) domain and a sequence that resembles the PSTAIRE cyclin binding region characteristic of cyclin dependents. Overexpression of Soc2 produces accumulation of cells with an unequal distribution of what appear to be paired chromosomes. Taken together the features of Soc2 suggest that this protein may have a direct or indirect role in chromosome segregation. Sister chromatid separation is a highly regulated process and defects may contribute to major genetic defects or cancer. Conserved amino acids in the UBA and TAIRE domains of Soc2 will be mutated to alanine and evaluated by overexpressing the mutants in wild-type cells. The Soc2 overproduction phenotype will be analyzed in detail by tubulin immunofluorescence and fluorescence in situ hybridization. Potential biochemical interactions between Soc2 and cell cycle regulators will be assayed by immunoprecipitation and Western blot analysis. In addition, condition allleles of Soc2 will be generated in order to assess the phenotype of cells that have lost the function of Soc2.